1. Technical Field
The present invention relates to a radio base station and a communication control method.
2. Background Art
A next-generation communication schemes such as a Wideband Code Division Multiple Access (W-CDMA) scheme, a High Speed Downlink Packet Access (HSDPA) scheme, and a High Speed Uplink Packet Access (HSUPA) scheme, i.e., a Long Term Evolution (LTE) scheme is studied in the 3GPP, which is a group aiming to standardize the W-CDMA.
As a radio access system using the LTE scheme, it is highly expected to use an OFDMA (Orthogonal Frequency Division Multiplexing Access) scheme for a downlink and to use an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme for an uplink.
The OFDMA scheme is a multicarrier transmission scheme in which a frequency band is divided into a plurality of narrow frequency bands (sub-carriers) and data is loaded on each sub-carrier for transmission. The sub-carriers are orthogonalized on the frequency axis and densely arranged, as a result of which it is expected that high-rate transmission can be achieved and the frequency use efficiency can be improved.
The SC-FDMA scheme is a single carrier transmission scheme in which a frequency band is divided for each terminal and frequency bands different among a plurality of terminals are used for transmission. In addition to the fact that the interference between terminals can be reduced easily and effectively, a variation in transmission power can be decreased, and thus, the SC-FDMA scheme is preferable in view of low terminal power consumption, expansion of coverage, etc.
By using FIG. 4, an operation in which an uplink data signal is transmitted in the mobile communication system of the LTE scheme will be explained.
In the uplink in the mobile communication system of the LTE scheme, a radio base station determines a transmission format of a data signal, notifies a mobile station of the transmission format, and notifies the mobile station of a control signal instructing the transmission of the data signal.
When receiving the control signal, the mobile station transmits the data signal to the radio base station at a predetermined timing. In this case, the control signal is referred to as Uplink Scheduling Grant.
In FIG. 4, the predetermined timing is a sub-frame of “n+4”, where “n” denotes a sub-frame by which the control signal is transmitted. It is noted that the scheduling grant of the uplink is one of Downlink Control Information (DCI), and as its format (DCI format), “DCI format 0” is employed.
Now, generally, in the uplink in the mobile communication system, there is an upper limit (maximum value) on the transmission power of the mobile station. For example, the maximum transmission power of the mobile station in the mobile communication system of the LTE scheme is 23 dBm.
When the transmission power of the mobile station is subject to the limitation by this maximum transmission power, it is probable that realizing a required SIR at a reception end of the radio base station becomes difficult.
For example, the mobile station located in the cell end needs to transmit the uplink data signal by a transmission power so large as to enable compensation of a pathloss because the pathloss between the mobile station and the radio base station is large; however, when the transmission power is subject to the limitation by the maximum transmission power, it is not possible to sufficiently compensate the above-described pathloss, and as a result, it is difficult to realize the required SIR.
It is noted that for the problem that the required SIR cannot be realized by the above-described limitation of the maximum transmission power, there is a method of improving the SIR at the reception end of the radio base station by decreasing a transmission bandwidth of the uplink data signal, as illustrated in FIG. 5, for example.
That is, in FIG. 5, the transmission bandwidth is decreased in Case 2 as compared to Case 1, and as a result, a signal power per unit bandwidth is large. As a consequence, the SIR at the reception end of the radio base station is improved.
As described above, in an environment, such as a cell end, in which the transmission power of the mobile station in the uplink is subject to the limitation by the maximum transmission power, there is a method of improving the SIR at the reception end of the radio base station by decreasing the transmission bandwidth.
However, the above-described improvement method has the following problems:
For example, as illustrated in FIG. 6, in two adjacent cells (in an example of FIG. 6, a cell 1 and a cell 2), when the above-described improvement method is applied and the transmission frequencies of the data signal are the same (in the example of FIG. 6, when a transmission frequency of the data signal of a mobile station A of the cell 1 and that of the data signal of a mobile station B of the cell 2 are the same), there is a problem that these signals (in the example of FIG. 6, the signal of the mobile station A and that of the mobile station B) are interfered to each other. It is noted that as described above, the transmission power of the uplink data signal is large at the cell end, and thus, it is understood that the influence caused by the above-described interference is serious.